1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a sensing method using self-capacitance and mutual-capacitance alternatively to reduce touch noises.
2. Description of Related Art
The principle of touch panels is based on different sensing manners to detect a voltage, current, acoustic wave, or infrared to thereby detect the coordinate of a touch point on a screen as touched by a finger or other medium. For example, a resistive touch panel uses a potential difference between the upper and lower electrodes to compute the position of a pressed point for detecting the location of the touch point, and a capacitive touch panel uses a capacitance change generated in an electrostatic combination of the arranged transparent electrodes with the touching part of a human body to generate a current or voltage for detecting the coordinate of the touching part.
A typical capacitive touch system uses a sensor circuit to measure a capacitance change of a touch panel to thereby obtain the information of the touched positions and the like, so as to compute the coordinates of the touched positions. However, in the process of fetching touch data, due to noise interferences on the capacitive sensor circuit, touchpad unit, and even driver circuit, an external noise to ground interference, or internal noise interferences generated in the ICs, the touch data may be distorted and drifted, resulting in that noisy spots are presented, as shown in FIG. 1, or real touch point is disappeared or coordinate is shifted.
A typical projected capacitive touch sensing can be divided into self-capacitance sensing and mutual-capacitance sensing. The self-capacitance sensing indicates that a coupled capacitance is generated between a touch object and a conductor line, and a touch occurrence is decided by measuring a capacitance change of the conductor line. By contrast, the mutual-capacitance sensing indicates that a coupled capacitance is generated between two adjacent conductor lines when a touch occurs.
The self-capacitance sensing senses a grounded capacitance on each conductor line. Thus, a change of the grounded capacitance is used to determine whether an object is approached to the capacitive touch panel. The self-capacitance or the grounded capacitance is not a physical capacitor, but a parasitic and stray capacitance on each conductor line.
The mutual-capacitance sensing senses a magnitude change of a mutual-capacitance Cm to thereby determine whether an object is approached to the touch panel. Likewise, the mutual-capacitance is not a physical capacitor but a mutual-capacitance between a driving conductor line and a sensing conductor line.
The self-capacitance sensing may easily cause one or more ghost points, but the correct relative positions still can be detected. FIG. 2 schematically illustrates the ghost points of the self-capacitance sensing.
In this case, the self-capacitance sensing generates two ghost points, but the correct touch points on X and Y axes can be indicated. Namely, even the real touch points cannot be recognized, it needs only to separate two touch points from the four points in FIG. 2.
The mutual-capacitance sensing uses the driving signals at different time to simply detect the correct positions of the two touch points. FIG. 3 is a schematic diagram of the typical mutual-capacitance sensing. As shown in FIG. 3, the driving signals are outputted at different time such that the correct positions of each touch point can be found through the time differences.
However, when there are external noises or the panel receives noises, it is found that the mutual-capacitance sensing is sensitive to the noise interference on the same sensing line. Because the touch system is connected to the real ground system, the touch system can steadily and accurately output the coordinates of the touch points due to the touch media (such as a user's finger) being also connected to the real earth's ground system. However, when the touch system uses an independent power, which is connected to ground that is different from the real earth ground, this may cause the coordinates of the touch points to greatly shake or generate other noisy spots, so that the steady touch system is negatively affected, as shown in FIG. 4.
FIG. 5 schematically illustrates a model of the mutual-capacitance sensing, wherein the capacitance Cd indicates a parasitic and stray capacitance on a driving conductor line, the capacitance Cs indicates a parasitic and stray capacitance on a sensing conductor line, the capacitance Cm indicates a mutual-capacitance between the driving conductor line and the sensing conductor line, and the capacitance Cf2 indicates a capacitance obtained when a finger touches the panel.
As shown in FIG. 5, based on a change of the capacitance Cm, it is determined whether the panel is touched by the finger. Due to the mutual-capacitance Cm, which is a very small capacitance about 0.7 pF, a great amount of impedance is presented when a driving signal is inputted to the driving conductor line D1. If a finger touches, a noise is introduced via the finger, and the amplitude of the driving signal is relatively small due to the capacitance Cm with respect to an integrator. Thus, the output signal of the integrator is greatly influenced by the noise.
FIG. 6 schematically illustrates a model of the self-capacitance sensing, wherein the capacitance Cf indicates a capacitance obtained when a finger touches the panel, and the capacitance Cx indicates a capacitance on a grounded conductor line. The capacitance Cx is relatively greater than the capacitance Cm, such that the driving signal charges and discharges a large capacitance Cx. Thus, the noise affection to the self-capacitance sensing is smaller than the mutual-capacitance sensing.
To improve this, a filter circuit is typically used to filter external noises other than the driving signal so as to reduce the noise affection thereby steadily outputting the coordinates. Generally, the filter circuit is added before or after the integrator, as shown in FIG. 7. The filter circuit can be a low-pass, high-pass, or band-reject anti-noise circuit.
The filter circuit can be a combination of resistors and capacitors, i.e., a passive filter circuit, which can work well in a simple circuit system. However, for the passive filter circuit in a touch system, its solution is different because different factories have different designs on the passive filter circuit. Also, difficulties are presented on a slight capacitance change and smaller input voltage with respect to other circuits. The cited conditions are disadvantageous to noise cancellation. The filter circuit has a good effect on a number of expectable noises or skippable noise rates. However, for a small signal which is sensitive to noises, a slight noise can cause an incorrect data determination, or even the original small signal is filtered out. Thus, for the touch system, the use of filter circuit is not satisfactory.
To overcome this, some touch IC design factories increases the voltage of a driving signal to cope with the noise interference. However, such a way increases the power consumption and is not suitable for handheld devices. In addition, the circuit before the integrator is affected by noises which are difficult to be canceled, even the voltage of the driving signal is enlarged.
Another typical method adjusts the threshold to determine whether there is a touch point. FIG. 8 is a schematic diagram of adjusting a noise threshold in the prior art. The capacitive touch system uses tiny capacitance change to determine whether one or more touch points are presented. To avoid the touch points generated by an error determination due to the noise interference, the noise threshold is adjusted up or down to meet with the environment change. As shown in FIG. 8, the noise threshold in Design A is too lower, and thus it is adjusted up in Design B.
However, it is not easy to implement the adjustable noise threshold because the touch IC cannot clearly know the noise source. In addition, for adjusting the noise threshold with different conditions, the system has to be re-initialized to thereby adjust the parameters and obtain the best noise threshold. Accordingly, the system resources are relatively consumed.
In U.S. Pat. No. 7,719,367 granted to Krah for a “Automatic frequency calibration”, a frequency conversion driving way is employed to avoid the noise interference from different bands. As shown in FIG. 9, a driving signal is applied with three frequencies to scan the touch panel. For most noises, this is a good way to avoid the noises. In addition, the data with the three frequencies can be compared so as to increase the accuracy on subsequently processing the coordinates of touch points. However, since the noise source is not clear, such a way cannot avoid all noise frequencies. Furthermore, the three-frequency driving signal inevitably uses more system resources, resulting in reducing the report rate of the touch points and consuming more power.
Therefore, it is desirable to provide an improved sensing method for a capacitive touch panel to mitigate and/or obviate the aforementioned problems.